Measuring device



March 1954 RS. CHILDS 2,671,892

MEASURING DEVICE Filed Nov. 6, 1948 3 Sheets-Sheei 1 INVENTOR ATTORNEYSROBERT s. GHILDSS' March 9, 1954 R. s. CHILDS MEASURING D-EVICE FiledNov. 6; 1948 Fig. 8

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INVENTOR ATTORNEYS March 9, 1954 R. s. cHlLos 2,671,892

MEASURING DEVICE Filed Nov. 6, 1948 3 Sheets-Sheet 3 Fig. l0

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74 A IL INVENTOR ROBERT S. CHILDS BY/ ATTORNEYS Patented Mar. 9, 1954MEASURING DEVICE Robert S. Childs, South Sudbury, Mass., assignor, bymesne assignments, to Edward G. Martin,

Cambridge, Mass.

Application November 6, 1948, Serial No. 58,651

10 Claims. (Cl. 340-195) The present invention relates to improvementsin the measuring device described in my co-pending application SerialNo. 794,192, filed December 27, 1947, now Patent No. 2,650,352, datedAugust 25, 1953. More particularly this invention involves a means forreducing or eliminating undesirable capacitive coupling between theinput and output of the measuring device.

The aforementioned. device depends for its operation upon inductivecoupling between a rotor, to which in the preferred embodiment highfrequency voltage is applied, and a stator. For reasons explained insaid application a non-ferrous core is used for the stator and rotor andthe inductively induced voltage in the secondary member is a smallfraction of the voltage imposed on the primary. At the same time thewindings of the stator and rotor which oppose each other across a narrowair gap constitute a substantial distributed capacitance, and at thehigh frequency (preferably of the order of 100 kilocycles per second)used in this device, the capacitive coupling between stator and rotorbecomes correspondingly large; that is, the capacitive voltage appearingin the secondary output becomes substantial. As a result of the smallinductive voltage and of the high stator-to-rotor capacitive coupling,the capacitive component of the output voltage may be of such size as tomask the inductive voltage. Furthermore, the magnitude of the capacitivecomponent varies with the relative positions of the stator and rotor.The apparatus is primarily for the purpose of distinguishing the angleof rotor rotation by the magnitude of inductive output voltage. thelarge capacitive voltage is to render it difficult to determine thinductive voltage accurately.

The principal object of this invention is to redues or eliminatedistributed capacitive effects without impairing the inductive couplingupon which the accurate measurement of small angular displacementdepends.

With this object in view, the present invention comprises an improvementin the apparatus described in my prior application, said improvementconsisting in the use of a continuous rotor winding with externalconnections made across a diameter and, in addition, the splitting ofthe stator winding into a number of equal sections which may beparallel-connected. This arrangement results in eliminating or at leastsubstantially minimizing capacitive voltages.

Another object is the development of a selfsynchronous device which bymeans of simple However, the effect of switching may use the sameapparatus as both the coarse and fine measuring elements, both of whichare necessary in some servomechanism applications. The sectors of thestator according to this invention can readily be connected either toutilize the capacitive output voltage for coarse (sometimes calledlow-speed) measurement, or in a manner whereby the capacitive voltage issubstantially eliminated and the inductive voltage remains for the fine(sometimes called highspeed) measurements.

A still further object is to provide simple means for leading in theinput voltage to or leading out the error signal voltage from the rotorof the apparatus in such a way that no torque reaction is exerted on thedelicate prime mover to which the rotor may be attached for measurmentpurposes. The present invention contemplates means for eliminating theusual brush and sliprings with their resultant inaccuracy due tocounter-torque in the measuring device. Elimination of thiscounter-torque makes possible a servomechanism system that is able totake full advantage of the inherent precision of the multiconductor,non-capacitive device about to be described.

In the accompanying drawing, with the aid of which I will describe myinvention, Fig. 1 is a sectional elevation view of the device describedin my prior application; Fig. 2 is a detail view of the type of windingused on both rotor and stator of that device; Fig. 3 is a view of thewinding and connections according to this invention;

Fig. 4 is a view of the windings of the section-- alized member,preferably the stator, which is here shown as a two-phase device; Fig. 5is a graph showing the variation in capacitive voltage output with rotorangular displacement for the device of my prior application; Fig. 6 is agraph of the capacitive output voltage vs. rotor displacement for theclosed diameter-connected rotor of my present invention when employed inconjunction with a stator having a winding of the type shown in Fig. 2;Fig. 7 is a graph of the capacitive output voltage vs. rotordisplacement for the closed diameter-connected rotor when employed inconjunction with a multiple-section parallel-connected stator winding;Figs. 8 and 9 are circuit diagrams showing how a two-phase deviceaccording to the present invention would be used in a servomechanismsapplication, Figs. 8 and 9 showing the coarse and fine connections,respectively; Fig. 10 is a view, partly in section, of apparatusincorporating the present invention; Fig. 11 is a plan view of a part ofthe device ranged around the peripheries of the disks.

diametrically opposed points on the rotor.

the winding of Fig. 2,.

shown in Fig. 10; and 12 is the circuit diagram equivalent of the samedevice.

The illustrated embodiment of the invention (Fig. 1) comprises a primarymember and a secondary member 8, of which one, preferably the primary,is a rotor, while the other is a stator. As shOWIl in Fig. 1, the statorand rotor may comprise disks, preferably of glass, arranged face to facewith as small an air gap between them as possible. The rotor is suitablymounted on a hub or spider I0 which in turn is mounted on a shaft l2.The stator carries a deposited metal conductor indicated by heavy lines64 in Fig. 1 and the rotor carries a deposit It opposed to deposit Thewindings described in my prior applica' tion are shown in Fig. 2 forcomparison with the windings in accordance with my improvement shown inFigs. 3 and t. Both the stator and rotor deposits comprise grid-likestructures ar- The conductor l8 comprises a single conductor arranged inzig-zag or back-and-forth fashion whereby there is formed a successionof juxtaposed seriesconnected bars. In this form of the invention thebars are radially disposed. The individual bars are connected at theirends by short connectors 25. The deposition of the conduotor may beeffected in any desired way, as by evaporation of metal, such asaluminum, in the desired pattern determined by a mechanical orphotographic process, as will be understood by those skilled in the art.

As shown in Fig. 2 the conductors on both rotor and stator of the deviceof my prior application were inter-connected to form a loop broken atone point to lead out two terminal connections 2|. The changes made bythe present invention will be apparent from a comparison of Fig. 2 withFigs. 3 and 4 showing the conductor connections according to the presentinvention. The primary member (Fig. 3) which in servomechanismsapplications will preferably be the rotor, has the important differencesthat the winding is closed to form an unbroken 100p and 1 the terminalconnections 22 and 23 are made at The result of this change is to causethe capacitive output voltage to. be substantially a sinusoidal functionof the rotor angular displacement. As

will be explained, this fact can be put to use to provide a coarsesystem which uses the capacitive voltage for measurement.

It is, to be noted that the rotor connection are made in such a way thatthe current directions in successive conductor bars are the same as inInstantaneous current directions are shown by arrows in Figs. 2 and 3.In Fig. 3 the current comes from terminal 22 on a. conductor bar 24 andthen divides to follow two parallel paths toward the other terminal 23.A single conductor 26 leads to the terminal 23. Thus the conductors 24and 25 serve as joint conductors for the two paths. By this arrangement,currents in adjacent conductors are in opposite radial directions; moregenerally, any two conductors spaced by an even number have currents inthe same radial direction, and conductors spaced by an odd number havecurrents in opposite radial directions. give the same general inductiverelationship between rotor and stator bars as exists in the winding. ofFig. 2.

The secondary member, preferably the, stator,

has two. windings, designated generally P1 and P2,

The result is to each formed of zig-z-ag conductor bars having the sameangular bar spacing as the rotor. The purpose of the two windings is toprovide a twophase system, and to that end the bars of winding P2 areangularly displaced from those of P1 by one-half the angularbar-spacing. Both windings are in inductive relationship with the rotor,and accordingly the bars of each stator winding are only about half theradial length of each rotor bar.

It will be understood that the bars are prefferably great in number, sayof the order of 1000 for each winding, but only a relatively smallnumber are shown in the drawing.

One of the stator windings, say P1, is formed in three sections 28, 30and 32, each spanning One end of each section is connected to a commonterminal 9, while the other ends of the several sections are brought outto terminals a, b and 0, respectively.

The second-phase winding P2 is likewise formed in three sectionsv 3.4-,36 and 38. These three sections are all connected in parallel byconnections leading to terminals 40.

Considering now the winding P2, in which, all of the sections areconnected in parallel, it can be shown that this winding, when used witha rotor winding like that of Fig. 3, is insensitive tocapacitance-coupling and is responsive only to inductive couplingbetween the rotor and stator. To show this theoretically, consider firstthat the rotor and stator are capacitively equivalent to two conductingbands opposing each other across a narrow gap. Starting. with the case.of my prior application in which is used a single-path rotor like thatshown in Fig. 2 hereof, it can beshown mathematically that there isinduced a capacitively-coupled voltage, between stator terminals whichis represented by the curve E0 in Fig. 5. In that figure, a is theangular displacement of the rotor with respect to the stator. Theinductive voltage E1. is also show-n on this diagram and varies with 0in a manner depending on the bar spacing. The inductive voltage En isusually smaller than, and may be masked by, the capacitance voltage EC.

By substituting a rotor having a winding of the type shown in Fig. 3, inwhich the terminals are diametrically connected, so that there are, twopaths from one terminal to the other, the stator voltage E0 across anyportion of thestator becomes a function of 0, as shown in Fig. 6. In

Fig. 6, E0 is essentially a sinusoidal function of the angle 0.

With the rotor winding of Fig. 3 and a multiple-section stator winding,such as P2 each section. of the stator winding acts the same as a wholestator, so far as the capacitance-coupled voltage is concerned. Thecapacitive voltage in any stator'section will be zero when the diameterthrough the, rotor terminals bisectsthe stator section. Rotation, of therotor causes the: capacitive voltage to change to a maximum, back tozero, to a maximum in the reverse phase, and back to zero again, asshown in Fig. 6. Each of the sections goes through the same variationbut since successive sections are displaced in position, their voltagesare likewise displaced. In Fig. 7 the capacitively-induced voltages, inthe three sections are shown at Ec34,.Ec36 and EcIiB as functions of.

It will be understood, that the diagrams of Figs. 6 and 7 represent therelative magnitudes of the several alternatingvoltages as functions, ofangular: position. and that; these. voltages change in "time at thefrequency of the input voltage; in

other words, the three voltages shown in Fig. 7 are in the same timephase but differ in space phase by 120.

Since the sections 134, 36 and 38 are uniform and symmetrical, it can beshown that the presence of three equal voltages displaced in space phaseby 120 will result in zero capacitance voltage at the output terminals40, and that all currents due to the capacitance voltages are localizedwithin the stator sections. Thus the effects of capacitive coupling aresubstantially eliminated, and only the inductive voltage E1. appears atthe terminals 40.

Some residual capacitance efiects may appear because of unavoidableasymmetry of the wind- 'ings or departure of E from a true sinusoid, but

they are small. Furthermore, they are still further reduced because thediameter-connected rotor results in doubling of the current in thewindings by paralleling the two halves of the rotor. The currentincrease more than doubles the ratio of inductively-coupled voltage tothe capacitively-coupled voltages.

The foregoing explanation applies also to the winding P1 if .theterminals a, b and c are all connected together. Elimination ofcapacitancecoupling effects occurs because the three sections thereofact identically with those of winding P2.

On the other hand, if capacitance coupling effects are desired, they maybe obtained either by connecting the three sections of P1 in series orby utilizing the eifects of the windings independtance coupling effectsis desired, the use of a two-section stator is preferred for simplicity.However, another feature of the invention resides in the use of the sameunits for both coarse and "fine control, and for the coarse control, 11.

must be greater than 2 in order to obtain an output signal as a functionof angular position.

The feature of utilizing the units heretofore described for eithercoarse or fine control will now be described. It is well-known in theart of servo-mechanisms that best control is obtained from thecombination of two systems of diiferent speeds, i. e. different rates ofvoltage change with the same rate of rotor angular displacement, moreproperly referred to as coarse and fine controls. Theinductively-induced voltage E1. passes through a complete cycle with avery small rotation of the rotor; this voltage is well adapted for thefine-control system. On the other hand, the capacitive output voltage ofa device using the diameter-connected rotor (Fig. 3) and a statorwinding of the type shown in Fig. 2 passes through only a single cyclewith a 366 rotation of the rotor; this voltage is adapted for thecoarse-control system. Operation may thus be on the coarse control (thatis, on the system using the capacitive-coupled signal) to the limit ofits accuracy, which will necessarily be within the nearest quarter of acycle of the fine information; then the fine or inductive-signal systemtorque reaction for high-precision systems.

may be switched in for measurement within the particular quarter-cycleof the inductive output voltage selected by the coarse system.

The circuit diagrams of Figs. 8 and 9 represent a servomechanismutilizing two of the units heretofore described, and capable by simpleswitching of being converted from a coarse to a fine control system. Oneunit 5|] (which may be termed the transmitter) has a rotor I8 fed fromany suitable alternating current supply 5|, and a stator having thewindings P1 and P2. The other unit 52 (which may be termed the receiveror control transformer) is identical, but the corresponding rotorwinding and stator sections are primed in Fig. 8. The voltage across therotor of unit 52 is utilized as an "error signal which is used'forpositional control in a manner familiar to those skilled in theservomechanism art.

For a coarse control system, the capacitive variation of voltage in therotor winding P1 is utilized. The terminals a, b, and 0, respectively,of the two units are connected together. The windings P2 are notconnected into the system. With this connection of the windings P1, thesections 28, 28', the sections 30, 30 and the sections 32, 32' are allindependently connected. The winding sections of each stator constitutethree separate phases in the sense of space-phase displacement. Byreasoning similar to that applicable to synchro theory, it can be shownthat the voltage across the terminals of rotor I8 is an error signal,which is a function of the angular displacement between the two rotorsI8 and i8. This error signal may be utilized for control purposes, aswill be understood by those familiar with the servomechanism art.

To convert to a fine control system, the sections of each winding P1 areall connected in parallel by jumpers 54 and 56 tying together theterminals a, b and c of the windings P1 of the respective units. Thisresults in the elimination of capacitance efiects heretofore described.The windings P1 of the two units are connected together by a wire 58,and the windings P2 are connected by a wire 53. By reason of thehalf-bar displacement of the two windings P1 and P2 of each stator, thisresults in a two-phase system, whereby an error signal appears across I8as a function of the relative angular position of the rotors.

It will be observed that the coarse-control connections of Fig. 8provide a three-phase system in which each phase covers of the stator,while the fine-control connections provide a twophase system wherein thephases are separated by 90 electrical degrees (the spacing between twoadjacent bars representing electrical degrees). Thus the coarse systemexecutes a complete cycle upon 360" relative movement between rotors,While the fine system executes a complete cycle upon a relative movementof only twice the angular bar spacing.

As is apparent to those skilled in the art, the

rotors of devices 50 and 52 are usually made integral with therespective input and output devices, between which the circuit measuresthe error angle, or angular difference. Electrical connections must bemade to the movable rotor. The usual brush and sliprings impose too muchTo take full advantage of the accuracy of angular measurement that ispossible with the measuring apparatus herein described, substantially atorqueless take-off from the rotor is required. Figs. 10 and 11 showsuch a take off embodied in a measuring device.

acvraca The. error voltage-producing members of the apparatus are thestator 60 and the rotor 62. Windings of the type described earlier areplaced on the opposing surfaces 64. Another stator plate 66 ispositioned on the opposite side of rotor 62 from the first stator, thetwo stator plates being clamped to a supporting and spacing flange 10.On the opposing surfaces 68 of stator 66 and rotor 62 are placedidentical pairs of concentric rings of conducting material 12 and 14.These rings oppose each other across the air gap and can be made ofsufficient area to provide series capacitors through which to makeconnection to the rotor.

Fig. 12 shows the equivalent circuit diagram of measuring apparatus withcapacitive coupling rings. The series capacitors which provide thecoupling to the rotor winding are indicated at 12 and 14. The rotor canbe made with very small inertia, and the capacitive lead-in couplingpermits connection to the rotor without involving counter-torque. Withthe high frequencies applied to this apparatus, preferably of the orderof 100 kilocycles per second, this coupling is suflicient for practicaluse.

A further advantage accrues from eliminating slip-rings by theorientation of parts shown in Fig. 9. Although angular accuracy of thedevice described in my prior application is independent of small axialdisplacements between the rotor and stator windings, some change involtage gradient or sensitivity will occur. However, with the rotorcoupling capacities on the side of the rotor opposite from the measuringwindings, an axial shift of the rotor relative to the stator thatincreases. the gradient of the stator output voltage tends to reduce thevoltage applied through the capacitive lead-in rings and vice versa;thus, a compensating action is obtained.

It is to be understood that the form of my invention, herewith shown anddescribed, is to be taken as a preferred embodiment, and that variouschanges in the shape, size and arrangement of parts may be resorted to,without departing from the spirit of my invention, or the scope ofarranged in accurately spaced array, the conductors of the two membersbeing in inductive relationship and having the same angular spacing,each conductor on one of said members being series-connected with thenext adjacent conductor to form a closed loop in which adjacentconductors carry currents opposite in direction, said loop havingterminal connections at diametrically opposite points on the loop, andthe conductors on the other member being similarly series-connected in aloop opened at at least one point to permit terminal connections.

2. Apparatus for electrical measurement comprising two relativelymovable non-ferrous members, a large number of conductors extending backand forth across each of said members and arranged in accurately spacedarray, the conductors of the two members being in inductive relationshipand having the same angular spacing, interconnections on one memberseries-con- :necting adjacent conductors to form a closed loop in whichadjacent conductors carry currents opposite in direction and terminalconnections on the same member at diametrically opposite points on saidclosed loop, and interconnections on the other member series-connectingadjacent conductors to form n separate groups positioned 360/n degreesfrom one another, in which adjacent conductors carry currents oppositein direction.

3. Apparatus for electrical measurement comprising two relativelymovable non-ferrous members, a large number of conductors extending backand forth across each of said members and arranged in accurately spacedarray, the conductors of the two members being in inductive relationshipand having the same angular spacing, interconnections on one member toseries connect adj acent'conductors to form a closed loop in whichadjacent conductors carry currents opposite in direction and terminalconnections on the same member at diametrically opposite points on saidclosed loop, interconnections on the other member to series connectadjacent conductors to form n separate groups positioned 360/11 degreesfrom one another, in which adjacent conductors carry currents oppositein direction, and conducting members to connect the said n groups inparallel.

4. In apparatus for electrical measurement, the combination comprisingtwo relatively-movable windings each made up of a large number ofseries-connected conductors extending'back and forth in a plane, saidconductors of the two windings being in inductive relationship and atthe same angular spacing, each conductor of one winding being seriesinterconnected with its next adjacent conductor to form one closed loopin which adjacent conductors carry currents opposite in direction withterminal connections at diametrically opposite points on said loop; eachconductor of the other winding being series interconnected with its nextadjacent conductor to form three separate groups in which adjacentconductors carry currents opposite in direction spaced at degrees fromone another.

5. In apparatus for electrical measurement, the combination comprisingtwo relatively-movable windings each made up of a large number ofseries-connected conductors extending back and forth in a plane, saidconductors of the two windings being in inductive relationship and atthe same. angular spacing, each conductor of one winding being seriesinterconnected with its next adjacent conductor to form one closed. loopin which adjacent conductors carry currents opposite in direction withterminal connections at diametrically opposite points on said. loop;each conductor of the other winding being series interconnected with itsnext adjacent conductor to form n separate groups spaced at 360/11degrees from one another in which adjacent conductors carry currentsopposite in direction and the groups being. connected in parallel withone another.

6. In apparatus for electrical measurement, a winding comprising a largenumber of seriesconnected conductors extending back and-.iorth andarranged to form a closed conducting loop, in which adjacent conductorscarry currents opposite in direction and terminal connections atdiametrically opposite points on said closed loop to form two parallelconducting paths, the conductors connected to the terminals. formingjoint conductors. for both parts of the winding..-

7. In apparatus for electrical measurement, a

winding comprising a large number of conductors disposed radially atregular angular spacing in a single plane, conducting interconnectionsbetween conductors to form with said conductors a continuous and closedloop, and terminal connections at diametrically opposite points on saidclosed loop, to form two parallel conducting paths, the conductorsconnected to the terminals forming joint conductors for both parts ofthe winding, a second winding, in inductive relationship with, andmovable relative to, the first, comprising an equal number of conductorsdisposed radially at the same angular spacing in a single parallelplane, and conducting interconnections between conductors of the secondwinding to form with the conductors n separate groups spaced 360/11,degrees apart, each group comprising conductors series-connected so thatadjacent conductors carry currents opposite in radial direction.

8. In apparatus for electrical measurement, a winding comprising a largenumber of conductors disposed radially at regular angular spacin in asingle plane, conducting interconnections between conductors to formwith said conductors a continuous and closed loop, and terminalconnections at diametrically opposite points on said closed loop, anytwo conductors spaced by an even number of spaces, carrying currents inthe same radial direction, and any two conductors spaced by an oddnumber of spaces carrying currents in opposite radial directions, asecond winding, in inductive relationship with, and movable relative to,the first, comprising an equal number of conductors disposed radially atthe same angular spacing in a single parallel plane, and conductinginterconnections between conductors of the second winding to form withthe conductors 11. separate groups spaced 360/n'degrees apart, eachgroup comprising conductors series-connected so that adjacent conductorscarry currents opposite in radial direction.

9. Apparatus for electrical measurement comprising two relativelymovable members, each having a winding consisting of a large number ofconductors extending back and forth in accurately spaced array, theconductors being series-connected so that adjacent conductors carrycurrents of opposite direction, the conductors of the two windingshaving the same spacing, the windings being inductively coupled wherebyan induced voltage appears at the terminals of one winding when theother is excited with alternating current, and shunt means foreliminating voltages due to capacitive coupling between the windings byconnections within one winding.

10. Apparatus for electrical measurement comprising two relativelymovable members, each having a winding consisting of a large number ofconductors extending back and forth in accurately spaced array, theconductors being seriesconnected so that adjacent conductors carrycurrents of opposite direction, the conductors of the two windingshaving the same spacing, the windings being inductively coupled wherebyan induced voltage appears at the terminals of one winding when theother is excited with alternatin current, one of said windings beingdivided into a number of equal sections connected in parallel, saidsections having induced therein capacitively coupled voltages in spacephase, the currents due to said voltages being localized withing thesections.

ROBERT S. CHILDS.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 264,643 Edison Sept. 19, 1882 679,008 Steinmetz July 23, 1901714,786 Day Dec. 2, 1902 787,302 Latour Apr. 11, 1905 1,268,712 HarleJune 4, 1918 1,630,757 Perkins May 31, 1927 1,763,966 Tanner July 1,1930 1,937,375 Woodward Nov. 28, 1933 2,038,059 Reichel Apr. 21, 19362,400,619 Woodward May 21, 1946 2,401,344 Espely June 4, 1946 2,402,603Clark June 25, 1946 2,409,876 Martin Oct. 22, 1946 2,426,226 Labin Aug.26, 1947 FOREIGN PATENTS Number Country Date 5,562 Great Britain Feb.11, 1891 of 1890

